ima: Add parser keyword to the policy

hulk inclusion
category: feature
feature: IMA Digest Lists extension
bugzilla: 46797

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This patch adds the new policy keyword 'parser' to measure and appraise any
file opened by the user space parser, while the parser opened
digest_list_data_add or digest_list_data_del securityfs interfaces.

This ensures that all files processed by the user space parser are
processed without including the FILE_CHECK hook in the policy. With this
keyword it would be possible to have a policy to measure/appraise only
executable code and digest lists.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Tianxing Zhang <zhangtianxing3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>